Recently, a magnetoresistive sensor having a ferromagnetic thin film has been used for an accurate control of a magnetic encoder or an accurate control of a capstan motor for VCR and it has been necessary to use such a sensor for controlling a motor with high accuracy. FIG. 1 is a schematic cross sectional view of a capstan motor for VCR in which a conventional magnetoresistive sensor is incorporated. In this capstan motor shown in FIG. 1, reference numerals 16, 17 and 10' respectively represent a rotor yoke, a rotor magnet and a frequency-generating magnet (FG magnet) in which N poles and S poles are alternatively magnetized at small pitches and these elements constitute a rotor part. Reference numeral 18 stands for a stator coil for driving the motor and reference numeral 19 represents a case. Reference numeral 15 represents a magnetoresistive sensor which is fixed to a holder 15H made of a resin. The magnetic field detecting portion 15S is arranged facing the FG magnet 10' normally at a distance of about 100 .mu.m, and thus, the rotation control of the motor is performed by the output signal from the magnetoresistive sensor. Reference numeral 7 stands for a lead portion of the magnetoresistive sensor for electrically connecting the magnetoresistive sensor 15 and a printed circuit board 20.
The FG magnet is in general magnetized at small pitches, and correspondingly, the intensity of the generated magnetic field is small. For this reason, a desired output cannot be obtained if the gap between the magnetoresistive sensor 15 and the FG magnet 10' is large.
FIG. 2A shows a plan view of a conventional magnetoresistive sensor before it is attached to the holder made of resin, and FIG. 2B shows a sectional view thereof taken along the line A--A of FIG. 2A. Reference numerals 15S, 15M and 7 represent a magnetic field detecting portion of the magnetoresistive sensor 15, a molded portion for reinforcing lead-bonding portion and a lead, respectively. In the structure as shown in FIGS. 2A and 2B, the electrical connection between the lead and the sensor pellet is usually performed through the bonding with a solder. Moreover, an electrical short often arises and the lead is sometimes peeled off from the lead-bonding portions when they are in the exposed state. Thus, the bonding portions are reinforced by molding for the purpose of eliminating such inconvenience. The reinforcement of the bonding portions are in general performed with the aid of a resin such as epoxy resin and the thickness of the resin must be more than the thickness of the lead, and more specifically, it must be 200 .mu.m or thicker.
This sensor is arranged facing the FG magnet 10, which produces the magnetic field signals, as shown in FIG. 3A. Then, the minimum gap between the magnetic field detecting portion 15S and the surface of the FG magnet 10 becomes greater than 200 .mu.m even if the molded portion with a resin approaches the magnet to such an extent that the portion almost comes in contact with the latter. Recently, motors have been miniaturized and made highly precise, and correspondingly, the pitch of magnetization of the FG magnet has become smaller, hence, the intensity of the magnetic field produced by such a magnet has a very small value. Therefore, the desired output signal from the sensor cannot be obtained, as has been described above, if the gap between the magnetic field detecting portion 15S and the FG magnet 10 is as much as 200 .mu.m. As has been discussed above, a desired value of output signal from the sensor cannot be obtained unless the gap is in the order of 100 .mu.m or smaller.
For this reason, in the conventional technology as shown in FIG. 3B, an element is arranged so that a molded portion 15'M is kept away from the FG magnet 10. However, if the element is arranged in such a manner, a larger space is required below the rotor for avoiding the contact of the molded portion with the rotor. Therefore, this makes it difficult to reduce the thickness of the motor. In order to obtain the maximum output signal of the magnetoresistive sensor, it is necessary to face the entire region of the magnetic field detecting portion 15'S to the magnetized surface of the rotor. In general, it is sometimes observed that the rotor is shifted up and down during rotation in the order of about several hundreds of micrometers. Accordingly, to prevent the reduction in the output of the sensor, it is inevitable to keep a large distance between the lower end of the magnetic field detecting portion 15'S and the upper end of molded portion 15'M reinforced with a resin because the magnetic field detecting portion and the magnetized surface always face each other even if the rotor position shifts up and down. This would be a major obstacle in the miniaturization of the sensor element.
Another conventional magnetoresistive sensor is disclosed in Japanese Utility Model Application Laying-open No. 158966/1986. FIG. 4A is a plan view thereof and FIG. 4B is a sectional view taken along the line A--A in FIG. 4A. As shown in FIG. 4B, this sensor is designed such that a terminal electrode portion 21T is formed on a lower portion of a linear step formed on a silicon substrate 21. Reference numerals 21S, 21M and 6 represent, respectively, a magnetic field detecting portion, a molded portion and a wiring portion for electrically connecting the terminal electrode portion 21T and a lead 7. By making the step larger, this sensor can be designed such that the resin molded surface is not projected over the level of the surface on which the magnetic field detecting portion is formed, unlike the foregoing conventional sensor, even when the terminal portion is reinforced through molding with a resin. If the sensor is designed so as to have such a construction, the distance between the magnetic signal source such as an FG magnet and the surface of the sensor, or the position at which the sensor is to be arranged can arbitrarily be established without the restrictions observed in the conventional ones explained with reference to FIGS. 3A and 3B.
First, the production of an element having such a structure requires extra steps to make the linear or band-like portion before processing a substrate and subsequently, the element is prepared. In addition, it is very difficult to practically produce an element having such a structure because of the following problems. A first problem relates to the structure of the substrate. The band-like stepped portion must have a difference of level in the order of at least 200 .mu.m for limiting the surface of the molded portion to a level lower than that of the surface on which the magnetic field detecting portion is formed. As a specific example, Japanese Utility Model Application Laying-open No. 158966/1986 discloses that the stepped portion may be formed by etching an Si substrate. However, the formation thereof is very difficult. Such a stepped portion cannot be obtained through only one run of etching, and thus, it is necessary to repeat such an etching process over several tens of times. In other words, the etching process must be repeated over many times, and accordingly, the resulting substrate would be very expensive when taking the cost for the production thereof into consideration.
Another problem is the difficulty in processing a substrate. In particular, it is very difficult to carry out the phololithographic process and it is impossible to obtain a pattern with high accuracy. When it is intended to prepare the sensor element having a band-like stepped portion as shown in FIG. 4B, a terminal electrode portion must be formed on the lower portion of the stepped portion. However, with a large difference of level, it is impossible to form a pattern on the band-like portion of the lower step through the photolithographic process. Therefore, a sufficient mass-production and productivity cannot be ensured. Moreover, as the terminal electrode portion cannot be formed through the phololithograph process, a fine terminal pattern cannot be formed with high accuracy. Hence, the distance between the terminals and the size thereof are increased. Correspondingly the size of the sensor element is also increased. On the other hand, if the width of the sensor element is reduced, this conventional sensor element having the foregoing structure suffers from the problems of electrical short arising between the terminals and of the contact with the bonded wires since terminal electrode portions are present on the same line (or band).
As has been explained above, there has not been a proposal of an element having a structure in which the surface of the molded portion is not projected over the level of the surface on which the magnetic field detecting portion is formed, which can effectively be mass-produced, connected through the wire bonding technique, and has high reliability.